A MONOPOLY IS WHEN ONE COMPANY OWNS THE ENTIRE MARKET.
For example if you can only get your gas or cable from one company they have a monopoly it is in no way related at all to the ski boot industry.
They wouldn't be better of worse then full tilts or spks because people have different feet sizes as well as needs from their boots. However nike's production capabilities should allow them to produce boots at a lower cost then most of the competition causing supply to shift right. Demand for ski boots should stay the same because people don't care how they are made but the shift in supply will move the equilibrium price of boots down.
Therefore if Nike entered the ski boot market they could produce boots of the same quality as full tilt and spk, possibly at lower price that would benefit us; the consumer.